An Organic Light Emitting Device (OLED) comprises at least an organic layer which is put between electrodes, a voltage is applied between the electrodes to inject holes and electrons, which are allowed to recombine in the organic layer, thereby an light is emitted by a light-emitting molecule in the course of a transition from an excited state to a lower energy state. The Organic Light Emitting Device is used for lighting display, backlight and lighting applications.
Starting from the support, OLEDs are divided into three categories, bottom-emitting, top-emitting and transparent OLEDs. The bottom-emitting OLED uses a transparent or semi-transparent bottom electrode, the light is emitted through a transparent support. A top-emitting OLED uses a transparent or semi-transparent top electrode through said light is emitted, in other words the light is not emitted through the support. Finally, transparent OLED (T-OLED) uses transparent or semi-transparent electrodes on both sides of the device in such a way that the light can be emitted both through the top and the bottom electrode. As used herein, the expression “bottom electrode” is understood to denote the electrode which is the closest to the support.
In general, a bottom emitting OLED comprises at least one transparent electrode generally made from indium-doped tin oxide (ITO), a transparent support for supporting the transparent electrode and a reflective counter electrode which is generally made from calcium, silver or aluminium. The transparent support is made, for example, of glass, ceramic glass or polymer film. The refractive indexes of the different constituents of the OLED are in the range of 1.6-1.8 for the organic layers of the light-emitting device, 1.6 to 2.0 for the ITO layer, 1.4 to 1.6 for the supporting substrate and 1.0 for the outside air.
Organic light-emitting devices are manufactured with a good internal light efficiency. This efficiency is expressed in terms of internal quantum efficiency (IQE). Internal quantum efficiency represents the ratio between the number of photons obtained divided by the number of electrons injected. It lies in the order of 85%, even close to 100%, in known organic light-emitting devices. However, the efficiency of these devices is clearly limited by the losses associated with interface reflection phenomena due to the differences between the refractive indexes of the materials constituting the layers defining the interfaces. The losses as a result of reflection (R) occur at the interfaces and cause a reduction in external quantum efficiency (EQE). The external quantum efficiency is equal to the internal quantum efficiency minus the losses through reflection. As a result of all these combined losses, it is commonly accepted that the external quantum efficiency is generally in the range of 20% to 25% of the internal quantum efficiency.
Indium-doped tin oxide (ITO) is the material most widely used to form transparent electrodes. However, its use unfortunately causes some problems. Indeed indium resources are limited, which in the short term will lead to an inevitable increase in production costs for these devices. Moreover, because of the limited resistivity of ITO, it is essential to use a thick layer to obtain a sufficiently conductive electrode (i.e., an ITO layer having a resistance of around 5Ω/□ requires a thickness such that the absorption of the electrode is increased). Moreover, thick ITO is generally more crystalline, causing an increase in the microscopic roughness of the surface, which must then be polished occasionally for use within organic light-emitting devices. For display applications, such ITO material may have sufficient conductivity because the pixels size is small, typically on the order of 1 mm or less. However, the conductivity of such transparent ITO electrodes can be insufficient for applications that require much larger emitting area such as lighting panel. Furthermore, indium present in organic light-emitting devices has a tendency to diffuse into the organic part of these devices resulting in a reduction in the shelf and operating lifetime of these devices.
To solve these problems, different electrode structures have been proposed. Document WO 2008/029060 A2 discloses a transparent substrate, in particular a transparent glass substrate, having a multilayer electrode with a complex stacking structure comprising a metal conductive layer and also having a base layer combining the properties of a barrier layer and an antireflective layer. This type of electrode enables layers with a low resistivity and a transparency at least equal to the electrode of ITO to be obtained, and these electrodes can be advantageously used in the field of large-surface light sources such as light panels. Moreover, these electrodes allow the quantity of indium used in their fabrication to be reduced or even suppressed entirely. However, although an antireflective layer in the form of a barrier layer is used, the solutions proposed in document WO 2008/029060 A2 do not seek in any way to optimize the amount of light emitted by an OLED limiting the losses associated with interface reflection phenomena. Furthermore, a colour variation of the light emitted by the OLED depending on the viewing angle is also observed.